1. Field of the Invention
This invention generally relates to a process for producing acrylic acid by catalytic gas phase oxidation of propylene and/or acrolein (which may be sometimes referred simply to as propylene etc.) with molecular oxygen-containing gas. The invention particularly relates to a process for producing highly pure acrylic acid by efficiently separating by-products, particularly acetic acid, from the reaction product obtained by the catalytic gas phase oxidation. More particularly, the invention relates to a process for producing highly pure acrylic acid wherein water is added to the reaction product to obtain an aqueous solution of the product to which an azeotrope solvent is further added thereby removing by-products through the azeotropic phenomenon.
2. Description of the Related Art
The catalytic gas phase oxidation of propylene etc., with molecular oxygen-containing gas in the presence of a catalyst for oxidation provides a reaction product in the form of a mixed gas which contains, aside from intended acrylic acid, by-products such as acetic acid, formic acid, acetaldehyde, formaldehyde and the like and unreacted starting materials such as propylene, acrolein and the like. In known processes of industrially producing acrylic acid by catalytic gas phase oxidation of propylene etc., with molecular oxygen-containing gas, the mixed gas obtained by the catalytic gas phase oxidation is introduced into an acrylic gas absorption tower. In the tower, the gas is contacted with water for cooling and absorption, thereby obtaining an aqueous solution containing acrylic acid and the by-products such as acetic acid. Then, acrylic acid is separated and purified from the aqueous solution by distillation.
Among the by-products, acetic acid is undesirably produced in comparatively large amounts. Since acetic acid has a high distillation temperature (boiling point of 118.1.degree. C. under atomic pressures), the separation by distillation involves the problem that acrylic acid is liable to polymerize during the distillation and that the relative volatility between acrylic acid and acetic acid is so close that it is difficult to simply separate one from another by distillation. To avoid this, there has been carried out a process wherein an azeotropic phenomenon of a three-component system consisting of acetic acid-water-azeotrope solvent or a multi-component system is utilized. More particularly, the reaction product is contacted with water to obtain an aqueous solution, to which an azeotrope solvent is added, followed by distillation to distil off a mixture of acetic acid-water-azeotrope solvent from the top of a distillator and to collect acrylic acid from the tower bottom.
In Japanese Patent Publication No. sho 63-10691, toluene is employed as the azeotrope solvent. Although most of the acetic acid could be azeotropically separated, part of the acid would be left unseparated, resulting in the discharge from the bottom of an azeotropic separation tower along with acrylic acid. This leads to further separation of the unseparated acetic acid by use of an acetic acid separation tower, thus needing two towers including the azeotropic separation tower and the acetic acid separation tower.
In Japanese Patent Publication Nos. sho 46-34691 and 46-18967, azeotropic distillation is performed using ethyl acetate, butyl acetate, dibutyl ether, ethyl acetate, hexane, heptane, ethyl methacrylate, propyl acrylate and the like. The azeotrope solvent, acetic acid and water are distilled off from the top of the tower and acrylic acid is obtained from the bottom of the tower. However, our investigations have revealed that mere removal of high boiling points' impurities through distillation of acrylic acid obtained from the tower bottom according to these processes is not satisfactory with respect to the removal of acetic acid, such acrylic acid being unsatisfactory for use as an acrylic acid product.
It will be noted that the impurities other than acetic acid mentioned as contained in the by-products, e.g. formic acid (boiling point: 100.8.degree. C.), acetaldehyde (boiling point: 20.8.degree. C.), formaldehyde (boiling point: -19.5.degree. C.) and acrolein (boiling point: 52.5.degree. C.), are all low in boiling point and can be readily removed without resorting to the azeotropic treatment.